The present invention relates to the mixing of substances e.g. thermoplastic material or elastomeric materials, possibly under inclusion of fillers, accelerators, lubricants, etc. whereby particularly a mixer is charged with the components to be mixed therein to obtain a mixture which is as homogenic as possible.
Whenever components are to be mixed for any purpose it is usually required that they be mixed quite intimately to obtain a homogenic mixture. For this, a mixing chamber is charged, either in the beginning of a mixing step or during mixing if necessary or so provided for. The chamber contains rotating mixing elements constructed and operating to obtain a homogenic mixture. Of course, the construction of these elements depends on the nature and consistency of the charges.
Generally speaking, these rotating mixing tools will (1) crush fragmentizes and grind the (solid) charge to obtain a granular or powderous state or (2) soften the (liquidious) material so that the substance assumes low viscosity (masticating). These two steps can be deemed preparatory as they depend greatly on the original physical state of the respective raw material. Moreover, these preparatory steps may have to be carried out prior to mixing, involving the substances alone whereby, however, the mixing tool may already be used. Following preparation, the tools should intimately mix the granulated material, possibly under additional grinding of the particles, fragmenting and breaking them into smaller size to obtain a still more powdery state of at least some of the different components to be mixed, so that surface portions of freshly cleaved particles are brought into mixing engagement with newly broken up particles of another component and vice versa to obtain ultimately a very homogenic mixture.
Obviously, homogeneity of a mixture depends on the average particle size and its volume, and also on the size distribution; a mixture to be as homogenic as possible requires that particle volume and size be decreased as much as possible. The mixing operation requires expenditure of mechanical work, specifically shearing work, dominating for slowly running tools, but predominantly kinetic energy is consumed for fast running tools. The work expended and energy consumed in this process is converted primarily into heat due to internal and external friction. That heat has to be removed, basically from the outside and through the powder itself. Since powdery substances are usually inherently poor thermal conductors, the heat removal process is quite limited. Thus, uncontrollable and undesirable hot spots could arise inside of the mixture. Such hot spots may result in undesired chemical reactions. For instance, one of the components may be a component that cross-links if a particular temperature is exceeded. If in fact a local hot spot temperature exceeds that limit above which cross-linking begins the material undergoes locally a change in consistency not wanted at that point. Previously, it was practiced to sense a maximum temperature (below any critical limit) and to shut the mixer off to permit its cooling. Obviously, mixing has to continue to obtain the desired degree of homogeneity, because if mixing is to be stopped entirely just because the mixer became too hot, the mixing state becomes rather arbitrary. Moreover, the temperature of the material to be mixed can only be determined locally, or on the outside, but not everywhere inside of the substance. Since ambient conditions may vary they modify the heat removal accordingly.
The mixing of substances has generally been carried out on the basis of predetermined mixing times. Obviously, if the mixer has to be shut down, mixing has to be extended. One has also tried to use the motor power as a criterium for the duration of mixing, but that was not satisfactory.